The present invention relates to a mounting substrate on which a plurality of LSIs are mounted in an electronic computer, and more particularly, to a mounting substrate having a multi-layered wiring structure formed by a thin film process with insulating layers of an organic film composed of polyimide or the like.
In a mounting substrate used in an electronic computer, a problem arises when attempting to enhance the mounting density of many LSIs to thereby minimize a signal delay in the substrate, and to increase a speed of signals transmitted between the LSIs.
To cope with this problem, attention is paid to a mixed substrate composed of a thin film and thick film, which comprises a ceramic substrate on which a wiring layer of tungsten or molybdenum formed by a thick film process is laminated and sintered, a polyimide interlayer insulating film formed thereon, and a conductive layer of copper or aluminium formed thereon by a thin film process, and this mixed substrate is in a process of development. With this arrangement, a signal can be transmitted at a high speed in high density, because a dielectric constant of polyimide in a thin film wiring portion is smaller than that of ceramics, copper and aluminium of low resistance and a semiconductor process can be used.
However, the number of laminated layers of a thin film wiring layer must be increased to cope with an increase in the number of mounted gates per unit area, which is needed by the improvement in performance of computers.
Several technologies for forming thin film multi-layered wirings have been reported. They employ, however, a thin film process as the basic process thereof, in which conductive layers, through holes and polyimide layers are patterned on a ceramic substrate or silicon substrate by exposing and developing photoresist. This thin film process is suitable to miniaturize wirings, but has a drawback in that since this process is a so-called sequential laminating system by which conductors and through holes are individually made a long time is needed to form a thin film wiring composed of many laminated layers, and further an entire substrate becomes defective if the substrate is determined to be defective in the final process and thus yield is lowered and a cost of products is increased. Further, a resistance of wirings is suppressed to a low level in the thin film wiring, and thus when a width of the wirings is miniaturized, a thickness thereof must be increased to provide a given cross sectional area.
As a result, a thickness of a wiring layer is made to be the same or greater than a width of wirings, and thus a problem arises in that even if fluid polyimide varnish is used, flatness is difficult to obtain, an accuracy of a wiring pattern is deteriorated as the number of laminated layers is increased, so that the wirings are often cut off or short-circuited. Further, a problem also arises in that, since a ceramic substrate having I/O terminals and the thin film wiring portion of an underlayer are repeatedly heated and immersed in water, chemicals and the like, they are deteriorated at the boundary thereof and polluted by the ions in impurities, whereby the reliability thereof is lowered.
To solve these problems, for example, in Japanese Patent Application Kokai 61-40048, there is disclosed a package comprising an original ceramic substrate having I/O terminals and a separate substrate which has a thin film wiring formed thereon and connected on the original substrate by soldering. This package, which has the thin film wiring portion simply formed independently of the ceramic substrate, is fundamentally different from that of the present invention, in which a thin film portion is formed as a unit. Further, in this prior art, since a tentative substrate used to form a thin film serves as an element for forming the package as it is, a thin film wiring layer has a thickness which is unnecessarily increased and thus this is not preferable in view of the increase of a transmitting speed of a signal.
Further, Japanese Patent Application Kokai No. 63-274199 discloses another method by which polyimide films in which wirings are formed are laminated in one lot and through hole portions are thermally bonded under pressure.
This method is effective to reduce a throughput time, but a problem arises in that, since a polyimide thin film is treated in a laminating process, an aligning accuracy is difficult to improve, the number of contact points is greatly increased, and a connecting portion has low reliability.